In order to be suitable for the production of influenza virus vaccines and to induce a protective immune response in vaccine recipients, a candidate seed virus must have hemagglutinin and neuraminidase surface glycoproteins that match the predominant wild type virus circulating in the human population. However, many wild type strains of influenza viruses are relatively poor at replication; and, if used for vaccine production, may reduce the total amount of vaccine producible before influenza season or delay the availability of vaccine for commercial use. It is possible to reassort influenza viruses so that the progeny virus contains the two genes for the hemagglutinin and neuraminidase from a wild type donor virus and the internal six genes from another donor with high yield characteristics. Studies have been started to produce high yield reassortant viruses for vaccine production with the intent of using the same viruses to better understand the genomic features which are important in controlling virus replication in eggs. During the past year of start- up studies, emphasis has been place on developing 1) optimization of the coinfection ratio of the donor viruses for reassortment in eggs and in tissue cultures; 2) production and testing of ferret and sheep antisera needed for suppresssion of the high yield donor virus and selection of the high yield reassortants; 3) development of a limiting dilution strategy for the isolation of clones of desired reassortants in eggs; and 4) identification of the phenotype and genotype of the reassorting progeny by methods including antigenicity in serologic assays, identification of genome by RNA sizing in polyacrylamide electrophoresis, and amplification of unique gene sequences by polymerase chain reaction.